I. Field of the Invention
The present invention relates to wireless receivers and receive paths in a base station. More specifically, the present invention relates to an improved receiver system architecture for wireless base stations which achieves enhanced dependability by separating diversity reception paths.
II. Description of the Related Art
In the field of wireless telecommunications, such as various cellular, Personal Communication Services (PCS), and Wireless Local Loop (WLL) communication systems, many different communication standards exist. For example, Code-Division Multiple Access (CDMA) digital communications may be governed by either Telecommunications Industry Association (TIA)/Electronics Industries Association (EIA) Interim Standard IS-95 (series) for cellular systems, or by ANSI J-STD-008 for PCS systems. Additionally, Time-Division Multiple Access (TDMA) digital communications may be governed by the TIA/EIA IS-54, or by the European standard Global System for Mobile Communications (GSM). Furthermore, analog FM-based communications systems may be governed by the Advanced Mobile Phone System (AMPS) standard or a related standard such as N-AMPS. Other wireless communication standards also exist for both digital and analog modulation.
According to any one of the above standards, wireless base stations communicate signals to one or more wireless mobile stations, such as cellular phones, PCS phones, or WLL phones. The wireless base stations primarily serve as the wireless xe2x80x9cgatewayxe2x80x9d to the telephone system. In general, the wireless base station will be in communication with many mobile stations at one time.
The ability of the base station to operate when an internal software, hardware or other failure occurs is inherent to the base station architecture. The ability of the base station to continue to operate, either through xe2x80x9cswitching-inxe2x80x9d additional backup or properly working components or by operating in a xe2x80x9creduced capacityxe2x80x9d mode, is a measure of how well the base station architecture was designed.
For wireless communication systems, the system designer strives to design a base station architecture which is both cost-effective and highly reliable. One aspect of this is when a failure occurs at the base station, it does not result in loss of communications with the many mobile stations it may be serving. As a result, the system designer strives to connect the various base station components: front ends, receivers, demodulators, etc., in a manner which provides the best system reliability while still maintaining good performance, low cost, small size, low complexity, high degree of modularity, etc.
Wireless service providers who purchase and operate the base stations often specify a Mean Time Between Failure (MTBF) which represents the average amount of xe2x80x9cdowntimexe2x80x9d that is tolerable. Often, this MTBF will be expressed as a total allowable downtime per year. xe2x80x9cDowntimexe2x80x9d is frequently defined as when the base station is unable to communicate at all with any mobile stations. Most service providers are keenly aware of this downtime because it results in a complete loss of revenues from that base station for the duration of the outage. As a result, a service provider will generally prefer that if a base station subsystem or component fails, that failure should affect the operation of the base station in the least significant way. Thus, reduced capacity modes of operation or partial degradations in service are strongly preferred over total loss of service.
A common base station architecture 100 which does not have optimum redundancy is shown in FIG. 1. In FIG. 1, a pair of antennas 102A, 102B capture RF signals and provide them to RF front end 104. Antennas 102A, 102B may be used for diversity reception, a well-known receiving technique in which the signal of interest is better received and processed by virtue of having two antennas receiving signals which can be compared and/or combined.
RF front end 104 typically comprises various bandpass filters and low-noise amplifiers which perform some initial frequency selection and signal amplification. RF front end 104 outputs two amplified signals 106A, 106B which correspond to antennas 102A and 102B, respectively. Receiver 108 receives, downconverts, and performs intermediate-frequency (IF) processing on the amplified signals 106A, 106B, and generates received signals 110A and 110B which correspond to antennas 102A and 102B, respectively. Demodulators 112A-112N demodulate and perform IF and/or baseband processing on the signals 110A, 110B, thereby recovering the signal of interest from the RF signals received by antennas 102A, 102B. The architecture of FIG. 1 may be generalized to multiple receive paths, one for each sector being served by the base station.
In the architecture of FIG. 1, the RF front end 104 and the receiver 108 are single points of failure. That is to say that when either RF front end 104 or receiver 108 fails for any reason, it breaks the receive path from antennas 102A, 102B to demodulators 112A-112N. Thus, any failure of RF front end 104 or receiver 108 will result in total loss of service for the base station employing the architecture 100 of FIG. 1. A single failure path defined by RF front end 104 and receiver 108 exists whereby failure of any unit in the failure path will result in failure of the entire reception path. Namely, RF front end 104 and receiver 108 are both in the same diversity reception path and also in the same failure path.
A common improvement made to the base station architecture of FIG. 1 is to provide a separate, redundant receive path which can be switched-in when the primary receive path fails. This is implemented by providing duplicate components such as a duplicate receiver 109 coupled by bypass switches 107, 111 which connect RF front end 104 and demodulators 112A-112N to the duplicate receiver 109 when the primary receiver 108 fails. This is often referred to as providing xe2x80x9cN+1 redundancyxe2x80x9d where there are N primary operating components and 1 duplicate component in standby that can be switched in to take the place of any one of the N primary operating components when there is a failure. Note also that bypass switch 107 could be placed before the RF front end 104, and a redundant RF front end (not shown) could also be switched in.
In addition to the increased cost, size and complexity of providing duplicate components for the N+1 redundancy, the bypass switches 107, 111 introduced in the receive path can introduce further undesirable signal level losses, thereby degrading the receive path performance. For example, a typical signal level loss incurred when introducing a switch matrix into the receive path is approximately 0.2 dB to 0.5 dB. This can be very significant when the receive path noise figure is typically in the 3 dB to 6 dB range. In addition, the control circuitry hardware and software (not shown) needed to detect a failure and control the switches also adds complexity, cost, size, and power dissipation to the base station. One can also call into question the reliability of the switches themselves.
What is needed is a base station architecture which improves the overall base station reliability without adding significant complexity or cost.
The present invention is a novel and improved base station and receiver system for use in a base station which achieves enhanced dependability by logically separating the diversity reception paths into different failure paths. In one embodiment, the receiver system includes a first diversity reception path for receiving a first radio signal and a second diversity reception path for receiving a second radio signal. The first and second radio signals may be amplitude and phase shifted versions of the same information signal according to well-known principles of diversity reception. At least one demodulator compares and/or combines the first and second radio signals in a diversity reception manner. But the first and second diversity paths are logically separated into different failure paths. The receiver system may further comprise a distribution bus which provides the received first radio signal and the received second radio signal to the demodulator.
In this embodiment, the first and second diversity reception paths may each comprise first and second diversity antennas and first and second diversity receivers. The first diversity receiver is coupled to an output of the first diversity antenna and the second diversity receiver is coupled to an output of the second diversity antenna. Furthermore, a first RF front end circuit may be coupled to an output of the first diversity antenna, and a second RF front end circuit may be coupled to an output of the second diversity antenna. The first and second RF front end circuits filter and amplify signals received by the first and second diversity antennas.
In an exemplary embodiment, the first and second receivers generate in-phase and quadrature samples of signals received by the first and second diversity antennas. Other embodiments generate other received signal formats.
The receiver system described above is useful for application in a wireless base station. In particular, the receiver system described above, having a first diversity antenna and a second diversity antenna, may be used in a base station having a single or plurality of sectors with each sector supporting a single or plurality of frequency assignments. The base station may also have many first and second diversity receivers, with the first diversity receivers coupled to an output of each of the first diversity antennas and the second diversity receivers coupled to an output of each of the second diversity antennas, i.e. many diversity receivers sharing one or more common diversity specific antennas. Again, the first and second diversity receivers are logically separated into different failure paths. Additionally, each of the first and second diversity receivers may handle many frequency assignments.